The present invention relates to a charging device used in an electrophotographic process, and relates to an image forming unit and an image forming apparatus using the charging device.
In image forming apparatuses using an electrophotography process such as a printer, copier, facsimile or multifunction peripheral, a charging device is used to uniformly charge a surface of a photosensitive drum. There are several types of charging devices. A widely used charging device (i.e., a contact-charging type) includes a charging roller contacting the surface of the photosensitive drum and applied with a direct voltage.
The charging device of the contact-charging type has a disadvantage that a charging potential is likely to be uneven. To be more specific, the charging potential is likely to be uneven in an axial direction of the charging roller. Therefore, it has been proposed to form polishing grooves on a surface of the charging roller in a rotating direction of the charging roller to thereby reduce unevenness of the charging potential in the axial direction.
Further, as printing is repeatedly performed, the charging roller gradually becomes dirty. Therefore, it has been proposed to provide a cleaning roller that contacts and cleans the surface of the charging roller (see, for example, Japanese Laid-open Patent Publication No. 2010-54795).
However, in the conventional art, the surface of the charging roller may become worn by contact with the cleaning roller. In such a case, the charging potential on the surface of the photosensitive drum may become uneven, and printing quality may be degraded.